1. Field of the Invention
This invention relates to an improved polystyrene foam prepared with an endothermic agent and carbon dioxide as the blowing agent. This invention also includes a process for making this improved polystyrene foam utilizing the endothermic agent and carbon dioxide as the only blowing agent. This invention is particularly suited for making polystyrene foam sheet having a thickness of less then about 0.5 inch.
2. History of the Art
Polystyrene foam, particularly in sheet form, is at the present time made from any number of blowing agents which may possess or cause one or more of the following undesirable characteristics: volatility, flammability, poor thermoforming characteristics, brittle foam sheet physical properties, high cost or an adverse effect to the ozone layer. Examples of these blowing agents would include aliphatic hydrocarbons and fully or partially halogenated hydrocarbons.
Some have experimented with blends of carbon dioxide and other blowing agents such as aliphatic hydrocarbons or fully or partially halogenate hydrocarbons. An example of this can be seen in U.S. Pat. Nos. 4,344,710 and 4,424,287. These patents, however, state that the use of a 100 percent carbon dioxide blowing agent has not been successfully employed in continuous direct injection foam sheet manufacturing (Column 1, lines 42-45 and lines 49-55 respectively) due to the extreme volatility. Use of these materials is said to produce corrugation and surface defects.
Others have developed methods and apparatuses for metering a gaseous blowing agent, such as an atmospheric gas, into a molten resin charge, such as polystyrene, in an accurate and precise manner. One example of this can be seen in U.S. Pat. No. 4,470,938. The apparatus of this patent meters discrete uniform volumetric charges of gaseous blowing agent into the molten resin using a free piston-cylinder combination which is given notice power by the supply gas source.
Still others, in U.S. Pat. No. 4,455,272, have formed polystyrene foam by extruding polystyrene, a nucleating agent, a physical blowing agent, and water where the physical blowing agent and water are injected separately or together into extruder as a blowing agent. The product produced has small cells at junction points between membranes of the foam. The patent states that extrusion processes for producing such foam are well known in the art and are normally based on known prior art physical blowing agents such as lower hydrocarbons and halogenated hydrocarbons, including propane, normal and isobutane, all of the isometric pentanes and hexanes and halogenated hydrocarbons exemplified by Freon 11, trichlorofluoromethane and Freon 12, dichlorodifluoromethane, but that also in some cases chemical blowing agents have been used. It is also noted in the patent that the water provides an extra amount of vapor pressure for the production of low density foams and also produces a large amount of evaporative cooling. The amount of water used ranges from 0.5 to 5 weight percent based upon the resin. The patent also notes that the amount of talc should be in the range from 0.01 to 0.3 weight percent based on the resin. More than 0.3 weight percent produces a product with very small cell structures resulting in low compressive strength.
A typical current commercial polystyrene foam sheet extrusion process may use a tandem extrusion process (two extruders in series). The first extruder melts the polymer to produce a polymer melt. A high pressure metering pump would then deliver blowing agent to the polymer melt at the end of the first extruder where mixing is initiated prior to entering the second extruder where further mixing and cooling of the blowing agent and the polymer melt occurs. After exiting the second extruder the polymer melt then passes through and becomes a foam structure at an annular die. The foam structure, in the shape of an annular tube, is then stretched over a forming mandrel. The annular tube coming off the mandrel is then slit and opened to form a sheet which is then gathered on one or more rolls. The sheet is then aged and then thermoformed into a foam sheet packaging article.
The use of an endothermic agent and carbon dioxide as the sole blowing agent avoids many of the problems of other blowing agents and blends of other blowing agents with carbon dioxide. The use of carbon dioxide in liquid form avoids the problems of handling a gas.
The process of the present invention can produce a thermoplastic foam having a greater density and less corrugation than a comparable thermoplastic foam produced using only carbon dioxide. It can also produce, at a increased die temperature, greater quantities of a thermoplastic foam having a lower density with an equivalent corrugation to a thermoplastic foam in an equivalent process for producing an equivalently corrugated thermoplastic foam produced using only carbon dioxide. Also, under equivalent process conditions, the thermoplastic foam of the present invention has less corrugation than a comparable thermoplastic foam produced using only carbon dioxide at the same density.
Thus the invention is polystyrene foam blown solely with carbon dioxide and an endothermic agent, preferably liquid carbon dioxide, and the process necessary to make such polystyrene foam.
The invention is particularly suited for making polystyrene foam sheet having beneficial physical properties for thermoforming purposes.